docs: def _compute_pga_rock(self, C, dc1, sites, rup, dists):
          """
          Compute and return mean imt value for rock conditions
          (vs30 = 1000 m/s)
          """
          mean = (self._compute_magnitude_term(C, dc1, rup.mag) +
                  self._compute_distance_term(C, rup.mag, dists) +
                  self._compute_focal_depth_term(C, rup) +
                  self._compute_forearc_backarc_term(C, sites, dists))
          # Apply linear site term
          site_response = ((C['theta12'] + C['b'] * self.CONSTS['n']) *
                           np.log(1000. / C['vlin']))
          return mean + site_response